FIG. 7 is a block diagram showing a schematic construction of a typical conventional infrared data-receiving circuit 1. This infrared data-receiving circuit 1 is installed in an apparatus such as a portable information terminal, and used for receiving infrared data that matches the standard of IrDA (Infrared Data Association) 1.0.
A supply voltage Vcc is applied to the cathode side of the photodiode 2, and the photodiode 2 outputs a photocurrent corresponding to the intensity of infrared light that has been received from the anode to the preamplifier 3. The preamplifier 3 converts the photocurrent to a voltage, and the output is inputted to the non-inversion input terminal of the amplification circuit 5 through the coupling capacitor 4. The inversion input terminal of the amplification circuit 5 is connected to ground through the coupling capacitor 6, and the non-inversion input terminal and the inversion input terminal are respectively maintained at a predetermined dc-input level of the amplification circuit 5 by pull-up resistors 7 and 8.
The signal voltage vsig from the amplification circuit 5 is directly inputted to the inversion input terminal of the comparator 11 for carrying out a waveform-shaping operation, and also inputted to low-pass filter 12 on the first stage. The output of low-pass filter 12 is inputted to low-pass filter 14 on the second stage through the buffer 13. Low-pass filter 14 is constituted by an integrator having a capacitor 15, and each of these low-pass filters 12 and 14 integrates the signal voltage vsig to find the average value vav, and outputs it to the non-inversion input terminal of the comparator 11 as the threshold voltage vth. The comparator 11 sets the output signal vo to the output terminal 16 at low level when the signal voltage vsig is not less than the threshold voltage vth.
Therefore, supposing that the unit cycle is T with the pulse duty of, for example, T/4, and that the peak value of the pulse is vp, the threshold voltage vth is vp/4, and as illustrated in FIG. 8(a)-FIG. 8(b)-FIG. 8(c), the higher the signal voltage vsig becomes, the greater the threshold value vth increases. Thus, the pulse width of the rectangular pulse, which is formed by discriminating the level using the threshold value vth, is always made virtually constant.
In the infrared data-receiving circuit 1 having the above-mentioned arrangement, supposing that the limited communication range is, for example, 1 (m), the output current of the photodiode 2 at the limited communication range is, for example, several hundreds (nA); in contrast, the output current at close-range communication is, for example, several tens (mA). This requires that the infrared data-receiving circuit 1 should have a dynamic range of nearly one hundred (dB).
Further, although the photodiode 2 is not required to be so highly-sensitive in the case of the transmission of the infrared light through optical fibers and therefore, has pulses having fast pulse rises and falls, it is required to be highly-sensitive in the case of the spatial transmission of the infrared light, and upon receipt of a high input, the fall of the photocurrent becomes slow, thereby resulting in a problem of tailing. In other words, the photocurrent has the waveform shown in FIG. 9(a) at the time of intermediate-range communication; however, it comes to have the waveform shown in FIG. 9(b) at the time of close-range communication, resulting in a problem of rounded waveforms in falling.
Therefore, when the threshold voltage vth of the comparator 11 is maintained at the average value vav of the signal voltage vsig from the amplification circuit 5 regardless of the intensity of the inputted infrared light, the pulse width of the output signal vo tends to become quite larger than the actual pulse width upon close-range communication. In this manner, great irregularities occur in the output pulse width depending on the communication range, resulting in a problem of limitation of the communication range.
In this respect, the aforementioned IrDA1.0 system has a maximum data-transfer speed of 115 (kbps), and supposing that the unit cycle is T, the transmitting pulse width is 3T/16; therefore, even if it expands not less than three fold, the transmitting pulse can be reproduced.
However, in the IrDA1.1 standard as shown in FIG. 10, since the maximum data-transfer speed is as high as 4 (Mbps), and since the quaternary PPM (Pulse Position Modulation) is used, high precision is required for the output pulse width. This raises a problem in which the above-mentioned infrared data- receiving circuit 1 is not applicable.
FIG. 11 shows a block diagram of another prior-art infrared data-receiving circuit 21. Since this infrared data-receiving circuit 21 is similar to the aforementioned infrared data-receiving circuit 1, the corresponding parts thereof are indicated by the same reference numerals and the description thereof is omitted. In the infrared data-receiving circuit 21, the signal voltage vsig from the amplification circuit 5 is inputted to the level-detection circuit 22 so that its peak value vp is detected. In response to the results of detection of the level-detection circuit 22, the threshold-value voltage generation circuit 23 forms a shift value vsft that is lower than the peak value vp by a predetermined voltage v1 as the threshold value voltage vth, and outputs it to the non-inversion input terminal of the comparator 11.
Thus, the threshold-value voltage vth is allowed to vary in accordance with the peak value vp; thus, different from the aforementioned infrared data-receiving circuit 1, it becomes possible to generate a pulse for the output signal vo without being adversely affected by tailing upon receipt of a high input.
However, even in the infrared data-receiving circuit 21 shown in FIG. 11, although it reproduces a comparatively accurate pulse upon close-range communication, it cannot maintain the aforementioned predetermined voltage v1 upon intermediate-range communication, thereby failing to form the threshold-voltage value vth.
Therefore, an infrared data-receiving circuit 31 as shown in FIG. 12, which is modified so as to incorporate the features of the aforementioned infrared data-receiving circuits 1 and 21, has been proposed. Here, since the infrared data-receiving circuit 31 is similar to the aforementioned infrared data-receiving circuits 1 and 21, the corresponding parts are indicated by the same reference numerals.
In the infrared data-receiving circuit 31, after the signal voltage vsig from the amplification circuit 5 has been reduced by the predetermined voltage v1, that is, for example, by 300 (mV), in the level-shift circuit 32, its peak value vp is sampled by the peak-hold circuit 33, and then the result of holding is inputted to the comparison circuit 35 through the buffer 34 as a shift value vsft.
Moreover, the signal voltage vsig is inputted to the comparison circuit 35 as the average value vav through the first low-pass filter 12 and the buffer 13 as well as the second low-pass filter 14 and the buffer 40. The comparison circuit 35 outputs the higher of the two levels of the shift value vsft and the average value vav to the non-inversion input terminal of the comparator 11 as the threshold-value voltage vth.
Therefore, as shown in FIGS. 13(a), 13(b) and 13(c), the threshold voltage vth becomes the average value vav when the signal voltage vsig is comparatively low; and as shown in FIGS. 13(d) and 13(e), it becomes the shift value vsft when the average value vav becomes smaller than the value that is reduced from the peak value vp by the predetermined value v1, that is, when the signal voltage vsig is comparatively high.
In the aforementioned IrDA1.1 system, since the data-transfer speed is 4 (Mbps) as described earlier and since the duty per unit cycle T is a 1/4 as shown FIG. 10, the pulse width is 125 (nsec). Therefore, the aforementioned average vav is 1/4 of the peak value vp of the signal voltage vsig. Further, when v1=300 (mV) holds, as described earlier, the value h, which allows the threshold value vth to switch from the average value vav to the shift value vsft, satisfies the following relationship: EQU h/4=300 (mV) (1)
By allowing the threshold value vth to conform to the input signal level, it becomes possible to eliminate the aforementioned adverse effects due to tailing in the photodiode 2 as shown in FIG. 9(b), and consequently to make the pulse width of a pulse that has been discriminated and shaped by the threshold voltage vth virtually constant.
The above-mentioned infrared data-receiving circuit 31 can reproduce the pulse width of the infrared pulse in a virtually accurate manner within a predetermined limited-range communication; however, such an infrared data-receiving circuit 31 requires that the holding-use capacitor 38 and an integrating capacitor 15 installed in connection with the low-pass filter 14 should have sufficient capacities, for example, as large as 200 (pF). Consequently, to install many capacitors occupying such large areas raises a problem in construction of compactly integrated circuits.